Percussion drilling is used to create a long borehole via a plurality of elongate drill string rods coupled together end-to-end by interconnected male and female threaded ends. The well-established technique breaks rock by hammering impacts transferred from the rock drill bit, mounted at one end of the drill string, to the rock at the bottom of the borehole. Typically, the energy required to break the rock is generated by a hydraulically driven piston that contacts the end of the drill string (via a shank adaptor) to create a stress (or shock) wave that propagates through the drill string and ultimately to the base rock level. Conventional male and female threaded couplings are described in U.S. Pat. Nos. 4,332,502; 4,398,756; 4,687,368 and DE 2800887.
When the male and female threaded ends of neighbouring drill rods are coupled to create the drill string, the joint is typically subjected to bending forces during drilling. These bending moments fatigue the coupling and lead to breakage within the threaded portion of the joint. Typically, it is the threaded male spigot that is damaged and determines the operational lifetime of the coupling. U.S. Pat. No. 6,767,156 discloses a threaded joint between two percussive drill rods having conical guiding surfaces provided at the leading axial ends of the male and female portions in an attempt to achieve a secure couple and avoid damage to the threads.
However, conventional threaded couplings and in particular the respective threaded ends of existing drill rods comprise a male part of the coupling that has a smaller diameter than a main length of the rod. This provides a drill string having a uniform outer diameter but a compromised coupling strength that is typically weak during bending. Even where there is no bending (which is uncommon) the incoming compressive shock wave will create tension in the coupling due to reflection in the free end of the male part which may cause fracture in the highest stressed area. Accordingly, what is required is a drill string rod that addresses these problems.